In recent years, everything including paper documents is computerized. Accordingly, in order to store a larger amount of data in an apparatus or a medium for recording electronic data, efficient reduction in a data amount has been demanded.
For example, in recording data of an image where a plurality of different elements such as a character and a photograph are arranged in a recording apparatus or a recording medium, a technique to efficiently reducing a capacity of image data is important. Namely, a coding technique increasing an amount of reduction in data capacity with less image quality deterioration is important. In the following, a region within an image where elements such as a character and a photograph are displayed is also referred to as an image region. Here, a recording apparatus is implemented, for example, by a hard disk included in a personal computer, a copying machine or a multifunction machine. In addition, a recording medium is implemented, for example, by a CD-R.
Japanese Patent Laying-Open No. 07-212601 discloses a technique to perform image region separation in a unit of matrix (block) used in coding processing and to apply coding using the matrix to a result after image region separation (hereinafter, also referred to as conventional technique A).
In addition, Japanese Patent Laying-Open No. 07-212601 discloses a technique for efficiently separating a photograph portion in a unit of matrix (hereinafter, also referred to as conventional technique A). In conventional technique A, the photograph portion is coded (for example, JPEG (Joint Photographic Coding Experts Group)) in a unit of matrix and a character portion (a portion other than the photograph) is binarized, thus performing lossless coding (for example, run-length coding). In this manner, image data including a plurality of different image regions can efficiently be coded.
Japanese Patent Laying-Open No. 2004-187000 discloses a technique based on MRC (Mixed Raster Content) defined in ITU-T recommendation T.44 (hereinafter, also referred to as conventional technique B).
In MRC, image data including a plurality of different image regions such as a character, a line and a photograph is separated into three planes in total, i.e., a foreground plane, a background plane and a mask plane. Here, the mask plane refers to a plane indicating which of the foreground plane and the background plane is to be selected. MRC defines a scheme for individually coding the three planes.
In general, in an image coding apparatus employing MRC, the mask plane representing information for selecting between the foreground plane representing a character and a line and the background plane representing a photograph or the like is constituted of pixels.
In the image coding apparatus employing MRC, the mask plane is responsible for holding information on shape of a character and a line at high resolution for lossless coding at high resolution. In many cases, the foreground plane holding information on color of a character and a line and the background plane including a photograph or the like where deterioration due to lower resolution is not noticeable are subjected to lossy coding at resolution lower than the mask plane.
In addition, Japanese Patent Laying-Open No. 2004-187000 discloses a technique to lower color haze or the like that occurs when resolution of the foreground plane and the background plane is lowered, by devising a method of calculating a pixel value of a don't care pixel in the background image region in the foreground plane and a don't care pixel in the foreground image region in the background plane. Here, the don't care pixel refers to a pixel not referred to in reproduction of an image by combining data.
Meanwhile, in electronically recording the image data, selection of a data format (data form) is important. For example, if image data is coded, stored and decoded in one apparatus, an original data format may be employed. On the other hand, if coded and recorded data is transmitted between apparatuses, it is preferable to comply with a data format widely used in general.
For example, the data format complying with JPEG scheme is widely used in a personal computer, a digital still camera and the like. In addition, PDF (Portable Document Format) developed by Adobe (trademark) is also one of the widely used image data formats.
In conventional technique A, a character is binarized for lossless coding. Therefore, in conventional technique A, lossless coding of a colored character or a colored line present in the photograph image region is difficult, and a coding condition for a photograph image region is applied.
In other words, in conventional technique A, coding causes apparent deterioration in the shape of a character or a line, depending on a condition such as color. In addition, conventional technique A does not employ the widely used data format and suffers poor versatility.
In addition, in conventional technique B, the mask plane is responsible for holding the information on shape of a character and a line. Therefore, if the mask plane is coded, high resolution and reversibility are required, which leads to necessity for a large data capacity for holding the mask plane.
Moreover, though conventional technique B achieves lowering in color haze originating from the don't care pixel, it is difficult to completely avoid color haze.